It is known about electrolyte for a vanadium redox flow battery that when contaminants such as Si compound are mixed in the electrolyte in the production process, the electrolyte has a detrimental effect on a membrane of the redox flow battery. Japanese Laid-open (Unexamined) Patent Publication No. Hei 8-148177 (U.S. Pat. No. 3,085,634) discloses that when the contaminants are removed from the electrolyte by a method outlined below, the detrimental effect on the membrane can be avoided.
“After ammonium metavanadate is completely dissolved in a neutral or alkaline condition, it is acidified to deposit polyvanadium compound, so that the polyvanadium compound thus obtained is used as raw material of electrolyte.
Although the Si compound is mixed in the process of producing the polyvanadium, since the electrolyte is prevented from being gelled even in an acid condition by limiting the concentration of the Si component to 1,000 ppm or less, or preferably 500 ppm or less, durability of the membrane is improved.”
This conventional technique has the following problems, however.
During repeated charge and discharge operations, contaminants gradually clog carbon electrodes in the battery cell or are adsorbed on the carbon electrode. For example, ammonium-vanadium compounds are deposited in the electrolyte, so that a flow of or a circulation of electrolyte is hindered by those deposited compounds.
As a result of this, the battery gradually becomes less efficient and eventually becomes inoperative to charge and discharge.
Also, in the technique disclosed by Japanese Laid-open (Unexamined) Patent Publication No. Hei 8-148177 cited above, since NH4 is removed insufficiently, the battery efficiencies are reduced to eventually cause inoperativeness of charge and discharge of the battery, as mentioned above. Also, this publication discloses the limitation of the concentration of Si in the raw material of the electrolyte, but does not at all refer to the concentration of Si in the electrolytic solution produced by using such a raw material. Further, it defines only the limitation of the concentration of Si, but does not give any consideration to a quantity of electrolyte used for the battery and to a relationship between area of electrode and absolute quantity of contaminants.
It is an object of the present invention to provide electrolyte that can suppress reduction of battery efficiencies and capacities with increased charge/discharge cycles of the battery, a method for producing the same, and a redox flow battery using the same electrolyte.